The rapid growth in data traffic and bandwidth demand by the final users has led to a growing interest in urban/regional networks constituted by links that are non-compensated in chromatic dispersion on conventional single mode fibres (with dispersion of about 16-17 ps/nmkm). To allow high service flexibility at low costs, the development of urban networks operating in the so-called third window of optical communications (around 1550 nm) at a transmission bit-rate of 10 Gb/s requires the application of new optical solutions relative to those adopted for the transport network.
For example, modulation formats characterized by a narrow frequency spectrum, and hence intrinsically robust with respect to chromatic dispersion, are known.
The robustness of the format is the maximum cumulated chromatic dispersion (corresponding to a maximum distance) for which the optical signal-to-noise ratio OSNR needed to obtain a given BER (Bit Error Rate) performance (e.g., 10−6 or 10−12) has to increase by about 1 dB with respect to the quantity needed in back-to-back conditions (connection between transmitter and receiver without chromatic dispersion).
A first example is the use of the Phase Shaped Binary Transmission (PSBT) format, also known as duobinary. Pure duobinary coding entails the generation of a signal at three amplitude levels starting from a two-level NRZ signal through a delay equal to the duration of the bit. Duobinary coding can give rise to high dispersion robustness through the compression of the bandwidth of the modulating electrical signal. Through a single electric filtering stage with cut-off frequency slightly higher than one quarter of the transmission bit-rate and the use of a Mach-Zehnder external optical modulator, duobinary coding without delay lines is obtained. This solution enables transmission at 10 Gb/s on non-compensated links up to distances of about 180-200 km with low penalties.
An additional example is the Combined Amplitude Phase Shift (CAPS) format, as described, e.g. in E. Forestieri, G. Prati, IEEE Photonics Technology Letters, 16, pp. 662-664 (2004). It takes up the advantages of the PSBT duobinary format, achieving the compression of the power, spectrum and the phase shifts by means of a generator of pulses provided with energy even at the “0” and an appropriate coding of the binary sequence. If the signal were zeroed at the “0”s, the CAPS format would coincide with the ideal duobinary. The CAPS format is obtained from the Differential Phase Shift Keying (DPSK) format generated by means of a Mach-Zehnder modulator through a narrow optical filtering. The optimal band-pass optical filter has cut-off frequency at about ⅔ of the transmission bit-rate. Tolerance to chromatic dispersion varies according to the type and the order of the optical filter used and it can reach 180-200 km of non-compensated links.
The described solutions require, for the implementation of the format, use of external modulators, typically electro-optical.
Alternatively, particularly advantageous is the use of directly modulated lasers (DMLs) as transmitters: relative to the use of external Mach-Zehnder or electro-absorption modulators, they offer the advantage of small dimensions, low drive voltages, low cost and high output powers. Unfortunately, the frequency chirp associated with modulation in DMLs significantly limits transmission in links that are not compensated in chromatic dispersion on standard SMF fibres.
In the third window, use of DMLs allows, at a bit-rate of 2.5 Gb/s, acceptable performance up to an accumulated dispersion of about 2000 ps/nm (corresponding to about 120 km of SMF fibre). Although these performance levels are not sufficient for long distance networks, they are acceptable for example for urban networks, also of coarse-wavelength division multiplexed (CWDM) type.
At a bit-rate of 10 Gb/s, use of DMLs limits transmission over conventional fibre (16-17 ps/nm/km) to a maximum of about 10-20 km. A practical, realistic application of DML sources modulated at 10 Gb/s requires solutions that enable chirp effects and hence the bandwidth of the modulated signal to be minimized. Among the methods proposed in the prior art, we find:                transmission over low dispersion fibres (less than 10 ps/nm/km) or negative dispersion. Use of negative dispersion fibres can compensate the positive chirp that is typical of the DML;        dispersion-supported transmission (DST) techniques, where the conversion of the optical frequency modulation into intensity modulation is exploited together with an appropriate electric filtering. This solution naturally complicates the structure of the receiver;        use of a narrow optical filtering downstream of a DML laser (hereinafter referred to as a ‘filtered DML’): dispersion robustness is obtained in this case from the particular phase relationship between the adjacent bits resulting from the simultaneous presence of amplitude modulation and frequency modulation. For example, for a sequence at 10 Gb/s, a frequency modulation of 5 GHz is induced by means of the DML chirp, so that to bit “1” is associated a carrier with frequency 5 GHz higher than that of bit “0”. The phase of the carrier then increases linearly by π during bit “0”. In this way, two “1” bits separated by an odd number of “0” bits are in phase opposition. The closing of the eye due to the widening of the bit “1” accumulated by dispersion during the propagation is elided thanks to the destructive interference generated by the phase shift of π between the two interacting bits. The narrow optical filtering increases the extinction ratio, converting the residual frequency modulation into intensity in order to optimize performance. Such a solution allows transmission at 10 Gb/s on non-compensated links up to about 150-170 km.        
Since dispersion is a linear effect, dispersion compensators—operating as linear filters in the optical domain—may be added to the solutions described above.
For example, optical dispersion compensators (ODC) are optical filters with appropriate phase transfer function and hence appropriate group velocity dispersion which can be inserted in line to compensate dispersion by operating directly on the optical signal. See for example in C. R. Doerr et al., J. Lightwave Technol., 34, pp. 166-170 (2006).